Adhesively bonded laminates are employed to make electrical apparatus that rely on magnetic induction. Such apparatus include transformers, generators, motors, and the like. For example, the iron core of a transformer may comprise at least one main leg and yokes constructed of laminae of magnetic material (such as textured silicon steel or an amorphous alloy), the legs and yokes being joined at their ends with a 45.degree. oblique or rectangular abutment.
In using electrical steels in electrical applications, an objective is to reduce the energy loss associated with magnetization of the steel sheets. For example, the energy loss in the magnetic core ("iron loss") of a transformer can be decreased by optimizing the design of the core and decreasing the specific loss of the material of which the core laminae are made. The latter objective has been achieved to some extent by improving the electrical steel in terms of chemical purity, grain structure, and crystallographic texture. The design of the core can be optimized to some extent by employing thinner gauge electrical steel sheets, since a thin sheet provides a reduced path over which magnetically-induced eddy currents may flow.
Although metallurgical limitations presently dictate a sheet thickness lower limit of about 0.004 to 0.005 in., the practical lower limit for sheet thickness is, to a large extent, determined by the consumers of these sheets, who use the sheets to fabricate electrical apparatus. Thin gauge sheets are expensive to use because they are more susceptible to being damaged and because they require additional labor in the fabrication of various electrical devices. Moreover, the core building factor tends to increase as the space factor decreases and the sheets are stressed while being stacked. (Building factor increases as sheet thickness decreases because: (1) an increase in the number of laminae per core increases the reactive volume between layers filled with insulation and air gaps; (2) the steel is stressed while handling the thinner sheets; (3) an increase in the number of layers results in an increase in the number of joints per unit mass; and (4) an increase in the average length of a cut sheet per unit mass increases the relative volume of steel deteriorated along the cut edges and increases the interlaminar loss.) Such considerations are the reason most commercially available steel sheets are provided by steel producers within a thickness range of from about 0.009 to 0.015 inch (0.23 to 0.40 mm).
Thus, there is a gap, in terms of sheet thickness, between what steel producers could offer and what electrical device manufacturers (e.g., stacked core makers) are willing to use. By implementing the thinnest available steel, it is presently possible to decrease the average no load loss in a stacked transformer core by 7-20%. The potential improvement in core loss will increase as thinner steel grades become possible.
U.S. Pat. No. 4,277,530, Jul. 7, 1981, titled, "Electric Steel Lamination," discloses a laminated article for electrical applications that includes two sheets of electrically isolated electrical steel each having a thickness of less than 0.020 inch (20 mils). A major shortcoming of the process disclosed in this patent is that it requires bonding at the steel manufacturer's site. The bonding procedure is impractical because it requires the steel manufacturer to fully complete the bonding, including adhesive polymerization, before coiling/uncoiling the steel. Bonding of the bent laminae is not permitted because the residual stress resulting from such bonding adversely affects steel quality, particularly when the radius of curvature is less than 187 inches, which is typically the case when steel is coiled.
Another drawback of the invention disclosed in U.S. Pat. No. 4,277,530 is that it requires the use of a heat curable adhesive layer. This layer has a thickness of about 0.001 inch, which is 5% (assuming a sheet thickness of 10 mils) or more of the doubled steel thickness, resulting in a decrease in space factor of 5% or more. To one skilled in the art, it is clear that this is absolutely unacceptable, since each 1% decrease in space factor increases the power loss by 11/2 to 2%.
Moreover, thin and solid adhesive film, when pressed between two sheets of steel as disclosed in U.S. Pat. No. 4,277,530, becomes thicker near the center and thinner at the edges of the laminate. When compression is applied to the stacked laminate, the difference in thickness creates compressive stresses and decreases the laminate's mechanical stability and deteriorates its shape.
U.S. Pat. No. 4,413,406, Nov. 8, 1983, titled "Processing Amorphous Metal Into Packets By Bonding With Low Melting Point Material," discloses a method for making transformer cores with a metallic bonding material. The disclosed method requires that the sheets be heated and bonded together, and then allowed to cool so that the bond solidifies. This requirement is believed by the present inventor to be objectionable because it impedes (slows) production. Moreover, this patent fails to address ways to limit the decrease in space factor caused by the bonding process.
Noise is also a problem associated with transformer cores. For example, in a transformer with high and low voltage windings on the main legs of the core, it is well known that vibrations having a fundamental frequency of twice the commercial power supply frequency occur. These vibrations are transmitted through the iron core support and oil to the tank, and are then propagated into the air. Such vibrations are attributable to the magnetostriction of the magnetic material forming the core, which is a consequence of the extension or contraction of the magnetic material in the direction of magnetization with a certain magnetic flux density. The noise caused by the vibration of the iron core is conventionally suppressed by surrounding the transformer with a sound-absorbing wall of concrete or the like, or constructing the tank with double walls, or arranging noise-absorbing material or gas-contained bags of noise-proofing material between the double walls of the tank. However, none of these methods is efficient in terms of cost or effectiveness.